Resistance device



July 21, 1970 L. H. HARDY RESISTANCE DEVICE Filed April 22, 1968 FIG.

l4 (SURFACE) I l5(REFRACTORY COATING) |6(F|LAMENT CORE) FIG.2

FIG.3

INVENTOR. LAVERNE H. HARDY 3,521,213 RESISTANCE DEVICE Laverne H. Hardy, Tonawanda, N.Y., assignor to The Carborundum Company, Niagara Falls, N.Y., a corporation of Delaware Filed Apr. 22, 1968, Ser. No. 722,869 Int. Cl. H01c 3/00 U.S. Cl. 338296 3 Claims ABSTRACT OF THE DISCLOSURE A filament resistance element for use at high temperatures consisting of a nonconducting refractory body carrying at least one conductive refractory filament or refractory coated conductive filament.

This invention relates to an improved electric resistance device and more particularly to electric resistance devices comprising refractory-containing filaments designed for use at high temperatures.

Metallic filaments have been long used as heating elements in such devices as toasters, space heaters and the like. Such filament heating elements, however, are unsuitable for use at temperatures in excess of about 900 C. since the wire resistance elements are oxidized at such temperatures. A higher temperature nonmetallic heating element, in rod or strip form, commonly made from a refractory electrically conducting material such as silicon carbide, is generally used. Nonmetallic elements can be used at temperatures on the order of 110O C. to 1200 C. and as such are suited for use as igniters for various fuels such as natural gas or fuel oil. Nonmetallic heating elements have not been commercially accepted for use as igniters in appliances such as gas fueled dryers because of variations in resistance between individual elements and because of the relatively high cost of nonmetallic elements.

Accordingly it is an object of this invention to provide an electric resistance element suitable for use at temperatunes on the order of 1200 C. wherein the resistance of said element can be readily controlled as in a filament heating element.

Another object of this invention is to produce an electrical resistance device which is useful as an igniter for gaseous and liquid fuels.

The foregoing objects and advantages are achieved by this invention which relates to a resistance element comprising at least one oxidation resistant electrically conducting filament carried by a nonconducting body wherein the electrically conducting filament is provided with surfaces comprising an oxidation and high temperature resistant refractory material.

Further objects and advantages of this invention will be apparent from a consideration of the following description of the embodiments set forth below and from the drawings wherein:

FIG. 1 is a side view of an illustrative resistance unit made in accordance with this invention.

FIG. 2 is an enlarged cross-sectional view of an electrical conductive filament used in this invention.

FIG. 3 is a side view of another type of resistance device made in accordance with this invention having a portion thereof broken away to show the interior of said device.

FIG. 4 is an enlarged cross-sectional view of the resistance device of FIG. 3 taken at line 44.

Referring to FIG. 1 there is shown one embodiment of this invention which comprises a nonconducting, insulating body 11 on which is helically wound resistance filament 12 which is electrically connected at both ends United States Patent 3,521,213 Patented July 21, 1970 to terminal bands 13 which are connected to a suitable source of electric current not shown.

Insulating body 11 shown in FIG. 1 is of cylindrical cross-sectional configuration although other cross-sectional configurations such as triangular or rectangular may be used in this invention. When using a rectangular or triangular configuration for the insulating body it is highly preferable to provide insulating body with rounded edges so as to avoid unduly stressing or breaking the resistance filament. The insulating body 11 is preferably made of alumina although other refractory materials such as mullite, beryllia, magnesium oxide and steatite for example may be used with equal success. The main requisites of the insulating body are that it be electrically nonconducting, resistant to high temperatures, resistant to oxidation at high temperatures, and be substantially nonreacting with the resistance filament.

Filament 12 is provided with a surface 14 comprising an oxidation and temperature resistance refractory material. Referring to FIG. 2, surface 14 is provided by means of a refractory coating 15 over a tungsten, carbon or graphite core 16. In the embodiment shown in FIG. 2 both core 16 and refractory coating 15 are electrically conducting although it is within the scope of this invention to provide a nonconducting coating over an electrically conducting core. It is also within the scope of this invention to form filament 12 entirely from an electrically conductive refractory material.

The refractory composition used to form surface 14 on filament 12 must be resistant to oxidation at temperatures on the order of l1400 C. and preferably is resistant to thermal shocking and flame erosion. When used as the composition for the filament, the refractory material should have a positive temperature coeflicient of resistivity since it would be undesirable for the resistance of the filament to drop as the temperature increases. It is preferred that the electrical resistivity of the filament be on the order of 2m cm. to 39 cm. When used as a protective coating on the filament the refractory composition shoulld have the same properties of thermal shock resistance, oxidation resistance, flame erosion and high temperature resistance as are required when the filament is composed entirely of refractory material. In addition, however, it is desirable that the coefiicients of thermal expansion of the refractory material and the core material be relatively close to each other in order to avoid any breaking away of the coating from the core due to differences in expansion and contraction between the core and coating.

A preferred refractory material for use in this invention is silicon carbide. Silicon carbide is resistant to thermal shock and flame erosion and is oxidation resistant at the temperature range for which elements made in accordance with this invention are designed to operate. In addition silicon carbide has a coefficient of thermal expansion of about 5.1 10- at a temperature range of between 25 C. and 1000 C. which compares very favorably with the coeflicients of expansion of two preferred core materials, tungsten and carbon which have thermal coeflicients of expansion of 4.8 1-0- and 3 10*- respectively, when measured at the same temperature range. In addition silicon carbide has electrical characteristics than make it suitable for use as the resistant filament or as an electrically conducting coating over a tungsten, carbon or graphite core.

In the embodiment of this invention shown in FIG. 1 the resistance filament 12 comprises a tungsten core having a diameter of one-half mil and having a continuous silicon carbide coating thereon of about two mils in thickness. The thickness of the coating on the filament is a matter of choice, it being essential only to provide a continuous oxidation resistant surface on the filament.

In the embodiment shown in FIG. 1 the coating is substantially thicker than the core since it is desired to have the protective coating act also as a conductor. It is prefen'ed to have the overall filament diameter on the order of about 4 mils in order that the filament be relatively flexible for ease of wrapping. However, larger sized filaments or more brittle filaments can be used in another embodiment of this invention as will be described in connection with FIG. 3 below.

The oxidation and temperature resistant surface is provided on the filaments in any of several well-known methods such as for example sputtering, flame spraying, plasma deposition or pyrolytic deposition. Typical methods for providing a refractory coating on a filament are taught in U.S. Pats. 2,978,358 and 969,012. Illustrative methods for producing refractory filaments are found in U.S. Pat. 679,926 and 3,294,880. As mentioned above, the thickness of the coating is not critical, it being essential only that a continuous and substantially impermeable refractory coating be provided on the surface of the filament in order to prevent attack of the core by oxygen. A typical method for providing a silicon carbide coating on a filament is disclosed in U .8. Pat. 2,978,358.

In producing the embodiment of this invention shown in FIG. 1 filament 12 is helically wrapped around insulating body 11. During the wrapping operation the resistance of the device is periodically monitored and the winding pitch can be varied to utilize a greater or lesser filament length depending on whether more or less resistance is desired.

FIGS. 3 and 4 illustrate another embodiment of this invention wherein a plurality of resistance filaments 21 having oxidation and temperature resistant refractory surfaces 26 are carried within an insulating tube 22. The filaments may consist entirely of an electrically conducting refractory material or may consist of a conducting core 27 having a continuous refractory coating 28 thereon. The length of the filaments is dependent on the desired resistance, it being obvious that an increase in the number and/ or length of the filaments will result in an increase in the resistance of the device. The ends of the filaments 21 extend beyond the ends of tube 22 for making electrical contact with a source of electrical current.

In the embodiment shown in FIG. 3 electrical contact is made by means of stainless steel tubes 23 having an inside diameter slightly greater than the outside diameter of tube 22 surrounding the extending ends of filaments 21 and a portion of each end of tube 22. Tubes 23 are affixed to tube 22 by a suitable means such as a refractory cement threading, or by means of crimping. Electrical contact between filaments 21 and tubes 23 is achieved by crimping a portion 24 of each tube 23 around the extending ends of filaments 21 to form an electrical connection therewith. Suitable electric leads not shown are attached to the stainless steel tube contacts.

This embodiment of the invention is particularly useful when the resistant filaments are brittle or of such a nature as not to lend themselves to forming about an insulating body such as in the manner of FIG. 1. The filaments are carried .within the insulating body and there is no necessity for bending or otherwise forming the filaments.

In utilizing the element of this invention as an igniter, the device is positioned in contact with a stream of combustible material such as natural gas, fuel oil and the like and an electric current is passed through the resistance filaments. The passage of the current through the filaments causes a rise in temperature of the element due to the resistance heating of the filaments to a temperature above the ignition point of the fuel and combustion occurs. In positioning the element it is preferred that the portions of the device carrying the electrical contacts be outside of the combustion area.

In igniter systems there is normally a temperature sensor in close proximity to the igniter and responsive thereto to determine that the igniter is operable before releasing fuel into the combustion area. The device of this invention, when used as an igniter, lends itself to the combination thereof with a suitable temperature sensing element such as a thermistor by means of mounting the sensing element within the insulating body. Such a combination, with a different type of igniter, is disclosed in U.S. application Ser. No. 626,745, filed in the name of William T. Terrell and entitled Igniter-Thermistor Assembly, now Pat. No. 3,467,812.

Although silicon carbide is the preferred refractory material for providing an oxidation resistant surface on the resistance filaments it should be pointed out that other refractory materials are suitable for use in this invention such as for example alumina, molybdenum disilicide, and magnesium oxide. Such materials are temperature resistant, and oxidation resistant at temperatures on the order of 1100" =C.1400' C. at which the device of this invention is designed to operate. Coatings of these materials on suitable electrically conducting core materials such as carbon, graphite, tungsten, nickel-chromium alloys and the like can be accomplished by flame spraying, low energy sputtering and the like as described above for silicon carbide. Devices made in accordance with this invention can take any shape or form consistent with the use of resistance filaments and insulating bodies and the particular configuration of the device will be dependent on the application for which it is to be used.

Preferred embodiments of this invention having been described and illustrated, it is to be realized that modifications thereof may be made without departing from the broad spirit and scope of this invention as defined in the appended claims.

-I claim:

1. An electrical resistance device suitable for use as an igniter for combustible gaseous and liquid materials comprising at least one electrically conducting, flexible filament helically wound about a nonconducting body resistant to oxidation at high temperatures and electrical contact means for making an electrical circuit with said filament, said conducting filament being uncovered by a case and open to the material to be ignited and comprising (a) a core of electrically conducting material selected from the group consisting of carbon and graphite, and (b) a coating thereon which is oxidation resistant at temperatures on the order of 11-0O C. to 1400 C., comprising silicon carbide.

2. The device of claim 1 in which said core is carbon.

3. The device of claim 1 in which said core is graphite.

References Cited UNITED STATES PATENTS 3,157,541 11/1964 Heywang. 3,406,044 10/ 1968 Harris.

ELLIOT A. GOLDBERG, Primary Examiner U.S. Cl. X.R. 

